Currently much attention is devoted to polylactide (also referred to as polylactic acid and abbreviated as PLA). PLA is an aliphatic polyester, which in essence can be manufactured from renewable resources. Such manufacture may involve the fermentation of starch, sugar or other renewable organic substrates into lactic acid. PLA can in principle be synthesized by direct polycondensation of lactic acid (lactate monomers), which has the drawback that a high molecular weight is not easily reached. Therefore, PLA is usually prepared by ring-opening polymerization of lactide, the cyclic dimer of lactic acid. Lactide is usually manufactured by polycondensation of lactic acid into PLA oligomers, followed by de-polymerization of these oligomers by a so-called ‘backbiting’ mechanism in the presence of a suitable catalyst. After purification, the produced lactide can be converted into PLA of controlled molecular weight by means of a ring-opening polymerization reaction (ROP) in the presence of a polymerization catalyst. The latter method can be used to manufacture PLA of high molecular weight. Especially the compound stannous octoate or tin-octoate (Sn(Oct)2 or stannous bis(2-ethyl-hexoate) is well-known as a polymerization catalyst in the manufacture of PLA under industrial large volume conditions.
A method as described in the opening paragraph is known as such, by example form the European patent publication WO2009/121830-A1 in the name of the current applicant, in which the well-known SnOct2 is used as polymerization catalyst. When the polymerization conditions are properly chosen, high quality PLA can be obtained by means of the known method. Under such conditions, the use of Sn-octoate as catalyst in a lactide-to-PLA process results in a desired fast polymerization rate resulting in a polymer resin having a relatively high melt stability and low racemization rate.
Although the mentioned Sn-octoate catalyst may function well under optimized polymerization conditions, there appears to be much interest in alternative catalyst systems in order to broaden the possibilities in the manufacture of PLA grades having different or improved properties or characteristics. More specifically, there is a clear interest in Sn-free catalyst systems for polymerization of lactide into PLA. Such alternative catalysts should however be able to provide reaction kinetics comparable with the reaction rates achieved with the known tin-octoate. Additionally, the thermal stability of the PLA manufactured with such alternative catalyst systems should also be high, and preferably higher than reached with PLA produced with tin-octoate. Further, the alternative catalyst systems should not or hardly induce racemization of the lactoyl moieties in the polylactide.